Telephone system



Sept 22,1942 G. c. REIER ETAL 2,296,151

TELEPHONE SYSTEM Filed June 4, 1941 3 sheets-sheet 1` T0 EBA. LINE CKT COMMON T0 07H5@ ATTZ'NDANTS G c ,QE/ER `\Q ./NVENTOR; AL YADUP ATTORNEY Sept. 32, li G. C. REER mm..

TELEPHONE SYSTEM Filed-June 4, 1941 3 Sheets-Sheet 2 coNNEcrcR ccT c' Sept. 2, 11942., G. c. REIER E1 m.

TELEPHONE SYSTEM Filed Jupe 4, 194:1

3 SheebS-Sheet 3 hun m23 mwkwwmwgg /M/ENTORS G. c. RUE/e A. TRA

Patented Sept. 22, 1942 UNITED ST TELEPHONE SYSTEM George C. Reier, Westeld, and Albert Tradup, West Englewood, N. J., assignors to Bell Telephone Laboratories, Incorporated New York, N. Y., a corporation of New York Application June 4, 1941, Serial No. 396,510

11 Claims.

This invention relates to telephone systems and particularly to improved telephone apparatus and circuits for providing facilities for completing a plurality of simultaneous telephone connections between a` plurality of calling lines and a single called line.

The invention has for its object to provide improved facilities whereby a single loud-speaker line circuit is rendered accessible to a plurality of callinglines so that a plurality of talking connections between a plurality of independent substations and a single loud-speaker station may be simultaneously established.

ThisV object is attained in accordance with a feature of the'invention by terminating a loudspeaker line circuit in a plurality of sets of line terminals of a connector switch, and multipling the said connector switch terminal sets with corresponding terminal sets of other connector switches, thereby rendering the loud-speaker line circuit accessible, simultaneously, to a plurality of calling stations by way of the plurality of connector switches.

In accordance with a particular feature of the invention, the connector switch control cir-cuit is so designed that the talking connection too the loud-speaker line circuit 'is completed immediately upon seizure of the loud-speaker line by the connector switch employed in setting up the connection. In other words, the usual ringing of the called line together with the ringing trip features which are required of connector switches employed in extending connections to the regular type of substation are dispensed with.

Another feature of the invention precludes the possibility of the loud-speaker at the called station howling or singing when the number of lines connected to the loud-speaker line is Varied. More specifically, this feature of the invention resides in the provision of means associated with the .loud-speaker line which functions to maintain a condition of balance between the network and line impedances at the terminals of a hybrid coil, included in theloud-speaker line circuit, irrespective of the number of calling lines that `may be connected to the loud-speaker station.

These and other features of the invention will be readily understood from the following detailed description made with reference to the accomi' present invention, may be originated. The equipment shown in Fig, 1 is that generally found at an order turret of the type which finds eX- tensive use in brokerage houses, department stores, airports, and other similar establishments. It is to be understood that this showing is used for illustrative purposes only, since any other type of calling station will serve equally well.

Fig, 2 is a diagrammatic representation of a connector switch, which serves to extend calls, originated at the station of Fig. 1, to the loudspeakerY line circuit terminating therein; and

Fig. 3 is a circuit diagram illustrating the loudspeaker line circuit of this invention.

Before describing the operation of the system made up of Figs. 1, 2 and 3, a brief reference to the equipment illustrated in each of the figures will be made.

In Fig. l there is diagrammatically illustrated an order turret OT of the type which is parti-cularly suitable for use in brokerage houses, airports, etc., and at which calls incoming from a private branch exchange may be answered by the actuation of a key individually associated with the private branch exchange line, and at which outgoing calls may be originated. The order turret equipment consists essentially of a plug ended telephone set Il); a pair of jacks l I-I2 by means of which the telephone set may be connected to the attendants key-set; a plurality of three position keys I3, I4, I5 and I6; and a telephone dial Il. The keys Ill, I5 and I6 are used for talking purposes, while the key I3 is used for making the attendants position busy.

- When a key is up, in the normal position illustrated, the line individual thereto is not in use. Moving the key to the mid or talk position connets the attendants telephone set to the line, and if a key is moved down to the hold position, the call on the corresponding line is held and the attendants telephone set is disconnected and may be used on some other line.

There are shown three lines 20, 22 and 23 terminating at the order turret OT. Key I6 controls the regular order turret line 20 which appears, with other order turret lines, at a private branch exchange board (not shown) and is used only to answer incoming calls. Key I4 controls an overflow line 22 which appears at each of a group of from ve to ten order turrets. Should a call come into the private branch exchange when all the regular lines, such as line 20, to a given group of turrets are busy, the call would `be connected to the overflow line 22 to be answered by the rst attendant to become available. Key

I5 at the order turret controls a regular private branch exchange extension line, such as line 23, over which an order turret clerk or attendant may place outgoing calls. The dial l1 is provided for setting up connections which require dial service. Line 23, in the system to be described in detail hereinafter, permits the attendant to call various locations or stations equipped with loud-speaking equipment, such as the station shown in Fig. 3.

The connector circuit C (Fig. 2) over which calls are extended from the order turret OT to the loud-speaker line circuit of Fig. 3,Y is in general, similar to well-known connector circuits which control the operation of connector switches in extending a connection to a telephone line of the regular type. It distinguishes therefrom primarily in that it is designed to omit the regular ringing and tripping features and to cut through the connection immediately upon seizure of the called line.

The loud-speaker station B (Fig. 3) consists essentially of a loud-speaking receiver 25, an associated amplier 26 of any suitable design, a distant-talking transmitter 21, a visual signal 28, a key 30, a hybrid coil H and a plurality of balancing impedance networks 3|, 32 and 33.

The loud-speaker line circuit, designated by the numeral 34 terminates in three sets of line terminals a, b and c of the connector switch C of Fig. 2. These terminals are connected in multiple with corresponding terminals of other connector switches, such as the terminals a', b and c of the connector schematically represented by the block C. By virtue of the fact that the line 34 terminates in three sets of connector terminals a, b, c of connector C which are multipled to similar terminals of other connectors, it may be seized simultaneously by three lines, such as line 23,

which extend from three different order turrets of the type shown in Fig. 1. The sleeve terminal of each set of connector terminals is connected to the winding of a sleeve relay, such as relays 40, 4I and 42, which when operated, function to connect a corresponding impedance network 3|, 32 or 33 to the network terminals of the hybrid coil H of the loud-speaker line 34. Thus, as the number of lines that are connected to the loudspeaker line 34 increases, the number of networks connected to the network terminals of the L hybrid coil H is increased correspondingly. In this manner, a condition of balance is maintained between the line and network impedances regardless of the number of calling lines that are connected to the line 34. Loud-speaker howling is thus prevented.

The operation of the system disclosed in Figs. 1, 2 and 3 will now be described, and for this purpose Fig. 2 is placed to the right of Fig. 1, and Fig. 3 tothe right of Fig. 2.

When the order turret OT is occupied, the telephone set I would be plugged into the jacks Il and I2 in order that the attendant may be prepared to answer calls incoming to her order turret position. At jack contacts 46 therefore, ground is connected to the conductor 55 of the regular private branch exchange line 26, by way of the contacts B of key I3, contacts 48 of key I4, and contacts 49 of key I5. This ground serves to actuate a signal (not shown) at the private branch exchange as an indication to the operator thereat that the particular` order turret OT is occupied and in readiness to receive calls. At jack contacts 56, ground is connected to the lead I of the overow line 22 to indicate to the private branch exchange operator that one or more turrets in a common group are being occupied. At jack contacts 53, ground is also connected to a lead which extends to a supervisors cabinet and serves to operate a signal thereat as an indication to the supervisor that the order turret attendants position is occupied. With the telephone set ID in the jacks II and I2, the attendants transmitter is connected to the transmitter supply battery 5. The order turret attendant is now prepared to receive incoming calls.

When the private branch exchange attendant or operator receives the indication (ground connection to lead of line 20) that the order turret OT is occupied, incoming calls to the private branch exchange will then be connected to the line 26 by the private branch exchange operator when the attendant at turret OT is desired.

It will now be assumed that the operator at the private branch exchange (not shown) receives a call for an order turret attendant and extends the call to the turret OT by way of line 20. The private branch exchange operator applies ringing current to the ring conductor of line 2U which extends to the gas tube 2| by way of the conductor 41. The tube 2| ionizes in the well-known manner and starts to oscillate. This produces harmonics of the ringing current which are heard in the receiver of the attendants telephone set I0. The path for this signaling current may be traced from ground, condenser 56, contacts 51 of key I4, conductor 58, contacts 59 and 6|) of key I6, contacts 6I of key I5, ring conductor 62, right windings of coil 63, normal contacts of dial I1, tip conductor 64, contacts of key I5, contacts 66 and 61 of key I6, contacts 68 of key I4, condenser 69 to ground. Since the receiver of the telephone set I0 is connected across the lower portion of the right winding of coil 63, the ringing induction is received therein and the order turret attendant recognizes it as a ringing signal.

Upon receipt of the ringing signal, the order turret attendant actuates key I6, associated with the line 2U, to the talk position, whereupon the ringing tone is removed from the attendants receiver and a bridge is connected across the tip and ring conductors through the retardation coil 10 and the induction coil 63. This bridge, which functions to provide an indication at the private branch exchange that the call has been answered, may be traced from the tip conductor 1| of line 20, contacts 12 and 66 of key I6, contacts 65 of key I5, conductor 64, normal contacts of dial I1, upper right winding of coil 63, retardation coil 10, conductor 62, contacts 6| of key I5, contacts 60 and 14 of key I6 to the ring conductor 15 of line 20. The turret attendant is now free to converse with the calling subscriber over line 2|).

It will now be assumed that the order turret attendant is unable to furnish the information sought by the party on line 20 and finds it necessary to obtain such information from a clerk who is located at the station B (Fig. 3). Such a situation may arise, for example, at an airport or air lines terminal, when the calling party seeks information concerning ticket reservations or airplane schedules which can only be furnished by a clerk at a station such as station B.

The order turret attendant now actuates the key I6 to the hold position whereupon the retardation coil 11 is bridged across the line 20 to hold the connection. The connection of coil 11 across the line20 is effected by the contacts 12, 18 and 14 of key I6. With the connection on line 2|) held by means of the coil 11, the order turret attendant proceeds to establish a connection between her position and the station B over i the private branch exchange extension line 23. To do this the key I5, associated with the line 23 is actuated to the talk position, whereupon the conductors 64 and 62 with which the dial I1 is associated are connected to the line 23 by way of the contacts 3 and 4 of key l5 and the normal hold position contacts 2 and E. The attendant may now dial the designation assigned to the station B after having received the usual dial tone. matic switches may be involved in the completion of a connection to the desired line 34, but that only the last switch, that is, the connector switch C, is illustrated in the drawings. The operation of such switches, if required, is well known and need not be incorporated in this description. It willbe assumed that the connection has advanced to the connector C and it will now be described how this switch completes the connection to the line 34.

When the key I5 is operated to the talk position, the lead 55 to the private branch en change line 2li is opened at the key contacts (i9. This functions to provide an indication at the private branch exchange that the line is no longer receptive to incoming calls. When the dial I1 is moved off-normal, the induction coil 63 and retardation coil 1E) are shortucircuited by It is to be understood that other autothe off-normal contacts of the dial and the ring side of the attendants receiver is opened. The" ring side of the attendants receiver is opened to prevent the attendant from hearing the make and break of the dial. The dial functions in a well-known manner to effect the transmission of dial pulses which are used to control the operation of automatic switches, only the connector switch C and its control circuit being illustrated in detail in Fig. 2.

When the connector circuit (Fig.- 2)V is seized in the course of the completion of the connection originated at the order turret OT, relay it@ operates in a circuit extending from grounded tery, upper winding of relay Illil, conductor itl,

upper normal make-before-brealr contacts of relay H32 to the ring conductor i533, over the i closed loop at the order turret O'I7 tip conductor 15M, lower normal makebefore-break contacts of relay |92, conductor |55, to ground by way ci the lower winding of relay IM.

Relay it operated, operates relay lot in r obvious circuit. At its middle armature front contact, relay |66 grounds the` sleeve lead thus holding the preceding switches (if any) in their operated position. Relay itt also opens the circuit to the release magnet 29! of the connector normal springs VON, front contact and innerf most armature of relay It@z to ground by wav of the back contact and armature of relay |6353. The vertical magnet |98 steps the shaft and brushes of the connector switch to the desired level in a well-known manner. Relays I and |09 are vmagnet H3, armature and .back contact I4 of relay |02, armature and contact ||5of relay Il, conductor ||1, normal make-before-break contacts of relay H8; back `contact and armature of relay |69, vertical off-nor1na1 spring ||2 andl its associated spring contact, front contact and innermost armature `of relay IGS, to ground by way of the back contact and armature of relay |853.

Thenext series of interruptions from the pulse relay mi! causes the rotary magnet I I 3 to operate in parallel with relayV |20. Magnet ||3 operates in the circuit traced in the preceding paragraph, and relay I2ll operates in an obvious parallel branch extending from the armature contacts I I4 and |15 of relays |62 and IIS, respectively. Relay I2@ operates on the first interruption of the dial and is made slow-to-release in order to remain operated on dial interruptions. The rotary magnet H3 steps the connector shaft and brushes around to the terminals a of the called line 34.

Relay |2ii prepares the busy testcircuit and short-circuits the normal m-ake-before-break contacts of relay H8, through which the rotary magnet `circuit is` taken, so that the operation of relay I I3, while the sleeve brush of the connector switch C' passes over busy contacts, will not open the rotary magnet circuit.

On the completion of the dial interruptions for the last digit, relay |20, being slow in releasing, remains operated a short time after the brushes make `Contact with the terminals a of the called line 3A, `during which ay busy test is made. On the release of relay |20 (assuming line 34 is idle), relay IIB operates through its lower winding in a circuit extending from grounded battery, winding of sleeve relay 42 (Fig. 3) associated with the called line 34, conductor 9, sleeve terminal |22 of the terminal. set a, corresponding brush, conductor |23, inner upper armature and back contact of relay |20, conductor |24, lower winding of relayV IIS, conductor |25, back contact and outermost `armature of relay |13 to ground at thev outermost upper armature and front contact of relay |35. Relay .I I5 operates sufficiently to close its contacts I 21, whereupon the upper winding of relay ||6 is energized andthe relay fully operates; At its innermost lower armature and front contact, 4relay I It places a direct ground on the sleeve` terminal |22 of the connector C which serves to operate the relay 42 associated with the called line 34. The same ground also places a busy condition on the sleeve terminal in the multiple bank a'. Relay I I6 operated, opens the circuit to rotary magnet ||3 at contacts I I5, so as to prevent further stepping of the brushes if the calling party should again operate the dial.

\ It also prepares Ia reversing circuit to relay |610.

It will be noted that when relay IIS. operated to close its armature contacts |21, an obvious circuit was completed from ground to the winding' of relay |30,.so that relays IIB and |30 operate substantially simultaneously. In previous connector circuits of the general type described, relay |30 served as a medium for completing the connection of a machine ringing current source to the called line when relay H6 operated. The path for such ringing current extended by way of an auxiliary winding of relay |30 which is generally referred to as the trip winding, which, when a called party answered a call, functioned to operate relay |30 to disconnect the ringing current source from the called line. In the present case, the tripping winding of relay |30 and the machine ringing current source are not required and relay |30 operates substantially simultaneously with relay ||6.

In the description thus far advanced, the brushes of connector switch C have been moved `into engagement with the terminals of set a. as-

sociated with the called line 34 and relays |30 land IIS of the connector circuit and relay 42 of 'the called line have been operated. With relays A|30 and H6 operated, the talking conductors of "the connector circuit are extended to correspondi'ing conductors of the called line by way of the tip and ring brushes and terminals of the switch,

vIt is thus apparent that the talking connection is completed immediately upon the seizure of an .idle set of line terminals by the connector switch. It will be noted that the winding of relay 2, at

the called station B is normally connected across vthe tip and ring conductors of the line 34. When the talking circuit is closed, due to the operation of relays |30 and ||6 in the connector Circuit, relay |02 operates by way of both windings over the closed loop effected by the winding of relay 2 of the called line 34. With relay |02 operated, talking battery, if the calling station were a common battery telephone, or supervisory battery in this case is supplied to calling station OT through the windings of relay |00. This battery connection may be traced from grounded battery, upper winding of relay |00, conductor front contact and outermost upper armature of relay |02, contacts 202 of relay ll, to the tip conductor |04; and from ground, lower winding of relay |00, conductor |05, front contact and outermost lower armature of relay |02, front contact and outermost upper armature of relay l5 to the ring conductor |03.

At the called station B, the switch 30 would be normally operated, so that when the connection is completed to the windings of relay |02 in the connector circuit C, relay 2 would operate to actuate the lamp signal 28 in an obvious circuit. The lighted lamp 28 serves as an indication to the clerk at station B that there is a call waiting on the line 34. A path is also completed for the energization of the transmitter 2T. This path extends from grounded battery, filaments of resistance lamp 203, left winding of coil 204, resistance 205, right winding of coil 204, resistance 206, front contact and left armature of relay 2, to ground by way of switch 30. The transmitter 2'! is energized by the potential drop across resistance 205.

At its right armature, relay 2 opens a short circuit normally maintained across the input to the loud-speaker amplifier 25 by way of the switch 208. The switch 208 serves, as is obvious, as a volume control switch for the loud-speaker 25.

It will be noted that prior to the seizure of the line 34 by the connector switch C, the network terminals 2|0 and 2|| of hybrid coil H are open circuited. When relay 42 operated, as a consequence of the seizure of the line 34 and the consequent connection of ground to the sleeve terminal |22 of the connector C, the impedance network, schematically shown at 33, was connected across hybrid coil terminals 2|0 and 2|| to balance the impedance of the line which is now connected to the line 34 by way of the connector C. This condition of balance insures against howling of the loud-speaker 25 which would otherwise occur.

Upon noting the lighted signal 28, the clerk at station B recognizes that his line 34 is connected to a calling line. The order turret attendant may now speak over the established connection by talking into the transmitter of his telephone set i0. As hereinbefore stated, the transmitter battery 5 serves to energize the attendants transmitter at the order turret OT so that, when the attendant talks into the transmitter, speech frequency currents are set up in the coils 03 and are impressed upon the line comprising conductors S4 and 02. The speech transmission path extends from the conductor 64, contacts 3 and 2 of key I5, conductor |64, condenser 2|3, front contact and upper armature of relay |30, contacts |33 of relay H6, brush 2|4 and corresponding connector switch terminal of set a, conductor 2|5 of line 34, left middle winding of hybrid coil H, condenser 2 S, the primary of input transformer 2|? of amplifier 26, lower left winding of hybrid coil I-I, conductor 2|8 of line 34, connector terminal and corresponding brush 2|9, contacts |32 of relay H0, lower armature and front contact of relay |30, contacts 220 of relay H6, condenser 22|, conductor |03, contacts 5 and 4 of key I5 to conductor 52. The speech signals set up in the primary of transformer 2 l'i are induced in the secondary thereof, amplified by the amplier 26 and impressed on the loud-speaker 25 by way of the output transformer 222. Speech signals set up at station B by the use of the transmitter 2 thereat are produced in the two upper windings of hybrid coil H and, by induction, in the pairs or line and network windings. Those induced in the line windings traverse the same path just traced to the order turret OT to be reproduced in the receiver of the telephone set I0. Due to the balanced condition of the hybrid coil, as hereinbefore mentioned, the speech signals produced by the transmitter 2l are excluded from the signal transmission path eX- tending to the loud-speaker 25.

Should it be necessary for the established connection to be held while the order turret attendant again picks up the call on the line 20, or for any other reason, this may be accomplished by actuating the key l5, at the order turret OT to the hold position. This operation disconnects the attendants equipment from the line 23 and bridges the line conductors |04 and |03 with the impedance network 225. This network is designed so as to have an impedance characteristic similar to that of the transmission circuit at the attendants station, which it replaces when the key I5 is actuated to the hold position. Vhen desired, the connection to the called line 34 may again be picked up by the order turret attendant by actuating the key i5 to the talk position. The results of this key operation are obvious.

At the termination of conversation between the order turret attendant and the clerk at station B, the connection is released when the attendant restores the key |5 to normal position. This operation releases relay ||i0 in the connector circuit, whereupon relay |05 releases. Relay |00 opens the locking circuit to relay |53 which was established at contacts |2'I` of relay II6. With relays |00, |06 'and IZ'I'released, ytllereleasernagnetf200 is energized in a'circuit extending -from grounded battery, windingof release magnet 200, contacts 228 of off-normal switch VON, backcontact and outermost lower armature of relay IIG, back contactV and lowermost armature of relay |06,` to'ground at the backcontact andarmature of relay |00. The operation of release magnet 200allows the connector switch shaft to return to normalin the well-known manner. When the switch is returned'to-normal, the releasemagnet circuit isj opened at contacts' 220 of the vertical off-normal switch VON.

It will now be assumed ,that, while the-loudfspeaker line 34 is connected tothe order turret OT, a second attendant, ata position equipped in a manner similar to thatshown in detail, in Fig. 1, desires to converse with'the clerk atstation B. At the Hupper portion or Fig. 1V there is shown a block O'T which is 'intended to represent an order turret similar` to that shown `at OT and at the upper portion of Fig. 2 there is yindicated a block C which is intended to `represent a connector C similar in all details to theconnector C. The three sets of connector terminals a', b and c are associatedwith the connector C" and, as hereinbefore mentioned, the individual terminals thereof are multipled with corresponding terminals of the terminal sets a., b and c of connector switch C.

As in the previous case, the second attendant would throw the key, such Vas key I5, tothe talk positionV and proceed to dial the line 34;`

The connector C' functions in thev'same manner as does connector C, previously described. It, will be noted that the sleeve terminals andhunt terminals such as |22 and 230', of each switch are tied together, and also these 'terminals of each line `position of each switch yare multipled. Hence, when relay IIiv operated, as previously described to place a direct ground on the `sleeve terminal |22, this` ground-was alsor connected to the `.sleeve termina-1 |22 of switch C.

. When the connector C functions in response involve relays of connector C andthat such relays are given thensame designations as corresponding relays of connector-switch C.) The circuit for relay IIB is completed beforewrelay I2!)r has had time to release. released, relay |02 operates'in `ar circuit-extending from grounded battery, `upper winding of relay |02, normal make-before-break contacts of relay |30, resistance 23|, outer upper armature and .back contact of relay |20, contacts 332 of relay III6, brush 232A and corresponding terminal 230" of terminal set a of `connector C" to ground. Relay IIB now locks under control or relay |02 and the armature contact of rotary magnet `I I3 to ground at the outermost armature and front contact `of -relay |06. I

Relayl 02 operated', closes a circuit for the op` eration of relay I2-0 which extends from ,ground-A With relay I-I 8 edbattery, `winding of' relay'l20, contacts |15 of erelay IIS, Vconductor II'I, front contact and lowermost armature of relay I I8, innermost lower armature and front contact of relay |02 to ground. At its frontV contact and inner lower armature, relay |20 operates rotary magnet I I3 in parallel'with'itself; The operation of rotary magnet ||3 steps the shaft and brushes of connector switch C to the next set of line terminals b andalso releases relay IIS; The reason for having the locking circuit for relay H3 under control of rotary magnet I|3 is for the'purpose of locking relay ||8 until therotary magnet is fully4 operated, at which time the brushes are centeredon the terminals so as to prevent false operation'l due to the breaking of the circuitvat the above terminal before the mechanism has completed a-full step. Similarly, relay |02 is locked in 'series with resistance 236- under control of relay II8. The release of relay H3 allows thev rotary magnet I|3 to-release.

-If the sleeve terminal 240 of terminal set b is grounded, due to the busy condition thereof, relay IIB` again operates which again causes the rotary magnet I|3 to' operate and step the brushes tothe terminals ofthe next set c', relay |02, holding operated through resistance 30 to ground on terminal 240. This operation continues until an idle set of terminals is reached or the last set of terminals in a group is reached, when the circuit functions the same as described in connection with the'testing of abusy line. The' hunt and sleeve terminals of the last set of a group of line` terminals are not strapped together.

It will be assumed that the second set of terminals bf of connector C' is found idle. In this case,relay ||6 operates' and is followed by the operation of relay |30. With relays IIE and |30 of` connector C"` operated, the talking connection between the loud-speaker line 34fand the order turret 'OT isy completed. The attendant at order turret OT when connected tc the line 34 may wait, if he should hear conversation being carried on by the clerk at station Bwith another attendant, or he may break in on the conversation depending on circumstances.

When the line 34 is connected to by the connector C', relay 4| operates in an obvious sleeve circuit, and in a manner similar to relay 42 hereinbefore described, connects an impedance network 32 to the network terminals 2E0 and 2| I of the hybrid coil H.

IfV the connector terminal set b" had been found busy, thev connector C' would automatically advance tothe next set c and relay E0 would function to connect the impedancenetwork 3| in parallel with the networks 32 and 33. Thus, for each calling line that is connected to the loud-speaker line 34, acorresponding impedance vnetwork is connected to networkterminals 2I0` and 2|I of the hybridv coil H to preserve the necessary condition of impedance balance and therefore, to prevent howling or singing of the loud-speaker 25.

Should all the terminal sets a', b`I and c be found busy relay |02 `would release on the release of relay II8, it being understood' that the sleeve andhunt terminals of the last sete" of a group of line terminals are not strappedtogether. The `ground from the sleeve terminal of the terminal set c would again operate relay I I8 beforerelay |20 releases. On the release of relay |20,` relayv II8 is lockedoperated in a circuit extendingfrom grounded battery, winding of release magnet 20), winding or relay H8, armature and back contact of magnet H3, normal make-before-break contacts of relay 120, front contact and uppermost armature of relay H8, to ground at the uppermost armature and front contact of relay H16. With relay l I8 operated the interrupter 215 causes a busy tone to be supplied to the calling station over the tip conductor |04, by way of the innermost upper armature and back contact of relay IGZ and condenser 253. The circuit remains in this condition until the call is abandoned at the calling station.

As long as there is a calling line connected to the loud-speaker line 36, relay 2 will be operated and lamp signal 28 lighted. When there is no calling line connected to the line 34, relay 2 will be deenergized, as will be also the lamp 23 and transmitter 2l. Also, when relay 2 is unoperated, the input to amplier 26 is short-circuited.

What is claimed is:

1. In a telephone system, a loud-speaker line, a hybrid coil included therein having a pair of network terminals, an impedance network, an automatic switch having a set of terminals corresponding to said loud-speaker line, a calling station, means at said calling station for causing said automatic switch to seize the terminal set corresponding to said loud-speaker line, means immediately responsive to the seizure of said terminal set by said switch for completing a talking connection between said calling station and said loud-speaker line, and means also responsive to the seizure of said terminal set by said switch for connecting said impedance network to the network terminals of said hybrid coil.

2. In a telephone system, a loud-speaker line, a hybrid coil included therein having a pair of network terminals, an impedance network, an automatic switch having a set of terminals corresponding to said loud-speaker line, a calling station, means controlled from said calling station for causing said automatic switch to complete a talking connection between said calling station and said loud-speaker line by way of the terminal set corresponding thereto, and relay means responsive to the completion of a connection between the calling station and the loudspeaker line for connecting said impedance network to the network terminals of said hybrid coil. Y

3. In a telephone system, a loud-speaker line, a hydrid coil included therein having a pair of network terminals, an impedance network, an automatic switch having a set of tip, ring and sleeve terminals connected to corresponding leads of said loud-speaker line, means at said calling station for causing said automatic switch to seize the tip, ring and sleeve terminals connected to corresponding leads of said loud-speaker line, means automatically responsive to the seizure of said terminals by said switch for completing a talking connection between said calling station and said loud-speaker line, and means controlled over the sleeve lead of said loudspeaker line when said terminals are seized by said switch for connecting said impedance network to the network terminals of said hybrid coil.

4. In a telephone system, a loud-speaker line, a hybrid coil included therein and having a pair of network terminals, a plurality of impedance networks, a plurality of automatic switches each having a plurality of sets of terminals corresponding to said loud-speaker line, calling stations, means at one of said stations for causing one of said automatic switches to seize said loudspeaker line by way of one of its sets of terminals, means at another of said stations for causing another of said automatic switches to seize said loud-speaker line by way of one of its sets of terminals, and means responsive to the seizure of said loud-speaker line by each of said automatic switches for connecting a different one of said impedance networks to the network terminals of said hybrid coil.

5. In a telephone system, a loud-speaker line, a hybrid coil included therein and having a pair of network terminals, a plurality of impedance networks, a plurality of automatic switches, a plurality of calling stations, means at said stations for operating said switches to cause them to seize said loud-speaker line, means responsive to the seizure of said loud-speaker line by each of said switches for automatically completing a talking connection between said loud-speaker line and each of said calling stations, and means also responsive to the seizure of said loud-speaker line by each of said switches for connecting an impedance network to the network terminals of said hybrid coil.

6. In a telephone system, a loud-speaker line, a hybrid coil included therein and having a pair of network terminals, a plurality of stations, an impedance network corresponding to each of said stations, means controlled from said stations for completing separate concurrent connections between saidloud-speaker line and each of said stations, and means responsive to the connection of each of said stations with said loud-speaker line for connecting the corresponding impedance network to the network terminals of said hybrid coil.-

'7.` In a telephone system, a loud-speaker line connected to a first calling line by way of an automatic switch and having an impedance network corresponding to the calling line associated therewith, a second calling line, a secondv automatic switch, an impedance network corresponding to said second calling line, means controlled from said second calling line and including said second automatic switch for completing a talking connection between said loud-speaker line and said second calling line while said loudspeaker line is connected to said first calling line, and means effective incident to the connection of said second calling line to said loud-speaker line for connecting the impedance network corresponding to the second calling line in parallel with the impedance network corresponding to the nrst calling line.

8. In a telephone system, a loud-speaker line, a hybrid coil included in said line and having a pair of network terminals, a plurality of calling lines, means including automatic switches for establishing simultaneous connections between said loud-speaker line and all of said calling lines, an impedance network for each calling line, and means eiective incident to the connection of said loud-speaker line to each of said calling lines for connecting a corresponding impedance neiiwork to the network terminals of said hyblfid co 9. In a telephone system, a plurality of stations, a first and a second automatic switch, a loud-speaker line terminating in a plurality of multiple sets of terminals of each of said switches, a talking connection completed Abetween one of said stations and said loud-speaker line by Way of a set of terminals of said first switch, means eiective incidentvto thecornpletion of said connection for busying'a corresponding set of terminals of said second switch, means at another of said stations for causing said second switch to engage the busied set of terminals thereof, means responsive to the engagement of `the busied set of terminals of Said second switch 'completing a second talking connection between said loud-speaker line and said other calling station, and means responsive to the completion of each of said connections for altering the impedance characteristic of said loud-speaker line.

10. In a telephone system, a loud-speaker line, a first automatic switch, a second automatic switch, each of said switches having a plurality of sets of terminals in which said loud-speaker line terminates, each set of terminals of one switch being connected in multiple with a corresponding set of terminals of the other switch, a first calling station, a second calling station, means at said iirst station for causing a connection to be completed to said loud-speaker line by way of a set of terminals of said rst automatic switch, means at said second calling station for causing said second automatic switch to engage said loud-speaker line by way of a set of terminals multipled with the terminal set of said first automatic switch by way of which a connection to said loud-speaker line from said first calling station is completed, means responsive to such seizure for causing said second switch to automatically step to a second set of terminals to complete a connection between said loud-speaker line and said second calling station, and means responsive to the completion of each connection to the loud-speaker line for altering the impedance of said line.

11. In a telephone system, a loud-speaker line having a tip conductor, a ring conductor and a plurality of sleeve conductors, a hybrid coil included in said loud-speaker line and having a pair of network terminals, a plurality of automatic switches each having a plurality of line positions assigned to said loud-speaker line and each position including tip, ring and sleeve terminals, the tip and ring terminals of all positions of all of said switches being connected in multiple to the tip and ring conductors of said loud-speaker line and the sleeve terminals of corresponding positions of said switches being connected in multiple with a separate sleeve conductor of said loud-speaker line, a relay included in each sleeve conductor of said loudspeaker line, an impedance network for each relay, a plurality of calling stations, means controlled from one of said stations for causing one of said switches to seize said loud-speaker line by way of the tip, ring and sleeve terminals at one line position of said one of said switches, means controlled from the other of said stations for causing other of said switches to seize said loud-speaker line by way of the tip, ring and sleeve terminals at other line positions of said other switches, means responsive to the seizure of the tip and ring terminals at each line position by its respective switch for automatically completing a talking connection between the loud-speaker line and the calling station from which the switch is controlled, means responsive to the seizure of the sleeve terminals at said line positions by each of said switches for operating the corresponding sleeve relay, and means controlled by each relay for connecting a separate impedance network to the network terminals of said hybrid coil.

GEORGE C. REIER. ALBERT TRADUP, 

